Bright entangled photon pairs from a nanowire quantum dot

A bright entangled photon source with high fidelity and on-demand generation is an essential resource in the advancement of ambitious schemes in quantum information processing such as quantum repeaters for long distance... [ view full abstract ]

A bright entangled photon source with high fidelity and on-demand generation is an essential resource in the advancement of ambitious schemes in quantum information processing such as quantum repeaters for long distance quantum communication. While such an entangled photon source seems out of reach with current photonic technology, here we present a bright source of highly entangled photon pairs emitted from a quantum dot embedded in a nanowire waveguide with an entanglement fidelity of close to 90% and a brightness that is more than two orders of magnitude higher than state-of-the-art quantum dot-based sources. The quantum dot emits the on-demand and strongly entangled photon pairs, while the high refractive index of a nanowire waveguide around it guarantees the directional emission of the light and the tapered end makes the light extraction  much more efficient. By optically exciting two electron-hole pairs (called biexciton |XX> state) in a quantum dot, a two-photon cascade happens through the biexcitonic decay. The two decay paths are indistinguishable and the two photons will be polarization-entangled if the fine-structure splitting between the intermediate states (known as exciton |X> state) is smaller than the radiative decay. Although impressive brightness has been achieved, quantum dot entangled photon sources are still inferior to parametric down conversion sources. It is due to the fact that non-resonant pumping limits the indistinguishability and coherence of the emitted photons. In order to go beyond this limit, while moving towards on-demand operation we are currently applying a resonant two-photon excitation (TPE) scheme to enhance the brightness and entanglement fidelity as done for self-assembled quantum dots before. In this scheme the biexcitonic state is pumped into saturation in order to ensure the emission of a single pair of entangled photons per each excitation pulse. A shaped laser is used to pump the quantum dot with the energy which lies between the excitonic and biexcitonic energies. This ensures that excitonic and biexcitonic photons are emitted as pairs and the excitonic population is not transferred to the charged (trion) state before the radiative cascade is over. Hence, the generation efficiency of the biexciton increases considerably compared to the non-resonant excitation. We showed a resonant two-photon excitation in a quantum dot embedded in a nanowire for the first time and we are improving it further. We anticipate that this resonant excitation (TPE) scheme will result in the on-demand generation of indistinguishable entangled photon pairs with state-of-the-art brightness and fidelity.